Say, "I only advise you of one thing," which is: "There is no god but Allah." He says this to the polytheists: "That you stand for Allah, in pairs and singly," meaning one by one or two by two, "then reflect: there is no madness in your companion," meaning Muhammad has no madness. "He is not but a warner to you before a severe punishment."
Muhammad said: The meaning is that he warns you that if you disobey, you will encounter a severe punishment.
﴿قل إِنَّمَا أعظكم بِوَاحِدَة﴾ ب (لَا إِلَه إِلَّا اللَّه) يَقُوله للْمُشْرِكين ﴿أَنْ تَقُومُوا لِلَّهِ مثنى وفرادى﴾ أَي: وَاحِدًا وَاحِدًا، أَو اثنيْن اثنيْن ﴿ثُمَّ تَتَفَكَّرُوا مَا بِصَاحِبِكُمْ من جنَّة﴾ أَي: مَا بمحمدٍ من جُنُون ﴿إِنْ هُوَ إِلا نَذِيرٌ لَكُمْ بَين يَدي عَذَاب شَدِيد﴾.
قَالَ محمدٌ: الْمَعْنى: ينذركم أَنكُمْ إِن عصيتم لَقِيتُم عذَابا شَدِيدا.